Selective emitters in solar cells (e.g., an emitter having very highly doped regions under metal contacts and having moderate to highly doped regions in other areas of the cell) are desirable because they allow for improved electrical conductivity between the metal contact and a P- or N-type doped region of the solar cell without negatively impacting light absorption of the cell in other areas when compared to a conventional solar cell without a selective emitter structure. Such selective emitters generally increase the power efficiency of a solar cell by increasing the currency extraction from the cell thereby lowering the cost of power produced from sunlight. For example, a conventional solar cell 100, as shown in FIG. 1, utilizing a p-type silicon substrate 110 (e.g. a wafer, a ribbon, thin film, etc., of p-type silicon) generally includes an N+ doped silicon layer 120 thereon and a P+ doped region 115 on the backside thereof. The conventional cell 100 further includes contacts 130a-c and an anti-reflective coating 140 on the doped silicon layer 120, and a backside contact 145. However, with such a conventional solar cell, current extraction and power conversion efficiency is limited by relatively high contact resistance between the N+ doped silicon 120 and the metallization contacts 130a-c. In principle, increasing the doping levels in doped silicon layer 120 to improve contact resistance would seem beneficial, but increased light absorption, especially in the UV and blue part of sunlight, by a highly doped N++ layer in areas not covered by the metal contacts decreases the amount of charge carriers generated in substrate 110 upon light absorption, and hence reduces the power conversion efficiency of such a cell.
To address these challenges, a selective emitter cell 200, as shown in FIG. 2, may be employed. A selective emitter cell 200 generally comprises an intrinsic or P-doped substrate 210 and a P+ region 215. An N+ doped silicon layer 220 is formed on the substrate 210. In the selective emitter cell 200, the doped silicon layer 220 includes N++ regions 225a-c (which have higher N-dopant concentrations than areas in layer 220 not covered by metal contact) under and adjacent to one or more contacts 230a-c formed on the doped silicon layer 220. Like the conventional solar cell 100 of FIG. 1, the selective emitter cell 200 includes a backside contact 245 and an anti-reflective coating 240. The selective emitter structure improves contact resistance without significantly negatively impacting light absorption. This results in increased power conversion efficiency.
An absolute increase in power efficiency (e.g., of about 1 to 2%) over the conventional cell 100 in FIG. 1 having a global emitter (e.g., an emitter having a uniform dopant concentration across the surface of the solar cell) has been reported for the N+/P/P+ solar cell 200 of FIG. 2 having a selective emitter cell formed from a doped silicon nanoparticle ink. Although this increase in power efficiency provides a significant advantage, it is known that silicon nanoparticles may show poor sintering and morphology properties when subjected to thermal annealing or laser irradiation. Such properties often lead to porous and/or rough film formation. Rough and/or porous films have a negative impact on the quality of subsequently deposited layers (e.g., an antireflection coating, surface passivation, etc.) on the front surface of the solar cell and on top of the silicon substrate with the doped silicon emitter contacts. Consequently, the quality of films formed from silicon nanoparticles alone may not realize the potential maximum power efficiency of a cell having a doped silicon emitter.